Dead seabirds continue to litter the beaches of Norther California, Oregon and Washington state. The massive die-off has been ongoing for the last two months, and biologists are still stumped as to the exact cause.

"It tends to come in waves," Dave Nuzum, a wildlife biologist with the Oregon Department of Fish and Wildlife, told The Oregonian. "Each time you get a significant weather event, you're going to get a crush of birds."

Nuzum says dead birds are likely to continue washing ashore in the coming months. Die-offs in autumn and winter aren't unheard of, but this season's death toll is particularly high.

The casualties have mostly been isolated to a single species, Cassin's auklets -- a small, chunky bird that dives in the frigid waters of the Pacific for food and builds burrowed nests in the mud and crevices of seaside cliffs. Because only auks have been found deceased in large numbers, biologists are confident the problem is not systemic and that the local food chain is relatively healthy.

"We're not seeing a widespread eco-disaster here," Julia Parrish, Aquatic and Fishery Sciences professor at the University of Washington, told the Santa Rosa Press Democrat late last year. "We're seeing a spike of (deaths in) one species that's giving us clues, and the clues don't suggest that the bottom is dropping out of the ecosystem."

Though unconfirmed, most biologists believe the mass die-off is simply a result of overpopulation. Cassin's auklets had prodigious mating seasons the last couple of years, so the numbers of young, inexperienced birds competing for food is high.

Combine large numbers of young birds not getting enough to eat with cold temperatures and rough seas, and it's not necessarily surprising that so many are ending up dead or dying on the beaches of the Pacific Northwest.